Since NASA retired its space shuttle in 2011, the Russian Soyuz has been the only space vehicle capable of sending astronauts to the International Space Station (ISS).

Bridenstine, who is visiting Russia and Kazakhstan for the first time since his appointment as Nasa chief this year, observed the launch from Baikonur cosmodrome with his Russian counterpart Dmitry Rogozin.

NASA had issued a worrying tweeton Thursday morning saying: 'There's been an issue with the booster from today's launch.

NASA astronaut Nick Hague and Russian cosmonaut Alexey Ovchinin are in what NASA director Jim Bridenstine described as "good condition" after surviving an emergency landing after a booster failure on a Russian Soyuz rocket Thursday.

The ISS crew will do their best to perform spacewalks in the future, but "on other dates", Sergei Krikalev, senior official at Russia's national space agency Roscosmos, told reporters on Friday.

The crew reached the ground safely.

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The rocket had lifted off at 4:40 a.m. ET on a journey that was expected to involve four orbits of the Earth and take six hours.

Roscosmos's online stream of the launch cut out shortly after lift-off.

When asked about the accident, U.S. president Donald Trump said that he was "not at all worried" that Americans had to rely on Russians to go to space.

Russian Federation has said it will suspend all its manned flights to the International Space Station until the cause of the accident has been investigated. Hague and Ovchinin were set to be just the second two-man crew to travel to the space station in recent years.

Search and rescue teams from Russian Federation arrived as soon as the spacecraft touched ground to assist the astronauts.

NASA officials now must decide how or whether to maintain a United States presence on the $100 billion orbital research laboratory as Roscosmos investigates the cause of the rocket's malfunction.

In August, a hole appeared in a Soyuz capsule already docked to the ISS which caused a brief loss of air pressure and had to be patched.

"Even when a failure occurs, because of the engineering and the design and the great work done by folks in Russian Federation, the crew can be safe", he said.

"I fully anticipate that we will fly again on a Soyuz rocket and I have no reason to believe at this point that it will not be on schedule", the Nasa administrator said.

The mission failure is stunning for the Soyuz rocket, which only had two launch failures prior to Thursday - one in 1975 and one in 1983. For now, the two astronauts are safe as are NASA astronaut Serena Auñón-Chancellor, German astronaut Alexander Gerst and cosmonaut Sergey Prokopyev.

The capsule carrying the men then separated from the malfunctioning rocket and made "a steep ballistic descent to Earth with parachutes helping slow its speed".